Register singletons in api_bp2build

Singleton modules are not used in this workspace (they do not implement
`ConvertWithApiBp2build`). However, we still need to add them since
there is a check in `SingletonModuleFactoryAdaptor` that depends on the
singletons being registered. We do this for bp2build as well (see
`RegisterForBazelConversion`)

This was probably always an issue in api_bp2build, but became
incompatible as a side-effect of the refactoring in aosp/2287719
that removed ctx.Register() from the newContext function.

Test: m api_bp2build --skip-soong-tests
Change-Id: I9ba1df2cc1e8ec916635e5280c66f8df68d7e79b